Version 1 May 2006
Vertical speed is in feet per minute (fpm), often abbreviated as ROC (rate of climb) or ROD (rate of descent), and is measured by a variometer. Some variometers are graduated in knots rather than fpm – one knot is equal to 100fpm.
Direction and position
The compass rose indicates directions and is divided into 360 degrees. Degrees are subdivided into minutes (60 to a degree) and seconds (60 to a minute). A direction of 220 degrees, 40 minutes, 30 seconds is abbreviated 220 40' 30". Alternatively minutes may be shown to one decimal point, so the same direction would be given as 220 40.5’. Using a hand-held magnetic compass you are unlikely to be accurate to within even one degree.
True bearings ( T) are directions relative to the true North Pole, which is on the axis of rotation of the earth. An arrow pointing to true north is normally shown on maps.
A topographic map uses a grid of east-west and north-south lines to help in measuring and locating positions on the map. There is a quite small difference between grid north and true north which is usually ignored.
Magnetic bearings ( M) are directions as they appear on the compass, relative to the magnetic north pole. The magnetic pole is offset from true north, and so a magnetic bearing is usually different from the true bearing. A typical relationship between true north, grid north and magnetic north is shown below:
Magnetic variation is the amount by which a magnetic bearing varies from the true bearing. The amount of magnetic variation depends on your location; it also varies over time. A magnetic variation of 10 east means that magnetic north is 10 to the east of true north. The magnetic bearing will therefore be 10 less than the true bearing (eg
ABF Pilot Training Manual Part 8
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